This invention relates to vinyl aromatic polymers and, more particularly, to improving their physical properties by altering the molecular weight distribution in the manufacture of such polymers.
In a mass thermal process for the polymerization of vinyl aromatic monomer, especially styrene monomer, the polymer produced has a broad molecular weight distribution and a low number average molecular weight (Mn) compared to polymers formed by solution or suspension processes.
It has been observed that styrene polymers having an increased number average molecular weight and a narrow molecular weight distribution, as measured by the ratio of the weight average molecular weight (Mw) to number average molecular weight, show better processability characteristics when converted to shaped articles. Apparently, a narrow molecular weight distribution gives a high viscosity at high shear rate which leads to a higher extruder output. It is an object of this invention to increase the number average molecular weight in vinyl aromatic polymers produced by a mass thermal process.